In the assembly of semiconductor packages, greases, fillers and adhesives are used to attach covers and heat sinks to completed dies and to attach different parts of a package to each other. Because a semiconductor die heats with use, the different parts of a package, including the die will expand and contract. Any adhesive must allow for this expansion and contraction. As a result, greases and polymers are often used between parts. On the other hand, heat must be conducted away from the die so that it does not overheat during use. Greases and polymers are very poor heat conductors, but most heat conductors do not accommodate expansion and contraction between parts of the package.
Thermal interface materials (TIMs) are used to attach heat spreaders to a die and to attach heat sinks to a package. TIMs are designed to balance adhesion, flexibility, heat conductance, thermal stability, ease of use, and cost, among other factors. A variety of different formulations have been developed for different applications that feature different characteristics.
The thermal performance of the current polymer thermal interface material (PTIM) limits its use in products with demanding thermal targets. The heat flux through the silicon die in future products during operation is expected to increase as the sizes of transistors shrink and more transistors are packed into a given area. Solder thermal interface materials (STIMs) are used in products requiring more efficient heat removal than can be accomplished with current PTIMs, but STIMs are more expensive. Indium is a common ingredient in STIMs and the price of indium continues to increase. STIM also requires flux to apply, integrated heat spreaders with gold plating to connect with the solder, and silicon dies with backside metallization to connect with the solder.